The use of magnets for fishing out "junk", i.e. met al fragments, from wells is well-known. For example, U.S. Pat. No. 3,520,359 (Ehrlich) discloses a magnetic tool consisting of a non-magnetic cage which houses a plurality of permanent magnets. A passage is provided in the tool to allow circulation of fluid downwardly through the tool to wash away non-magnetic materials and thus facilitate the retrieval of metal junk by the magnet. A disadvantage of this type of device, however, is that the tool is suited for use in the retrieval of junk after it has fallen down the well rather than capturing the junk while falling.
Another type of device, exemplified in the disclosure of Canadian Patent Application No. 2,232,941 (Owens et al.), is a magnetic retrieval tool for use in a method wherein a section is cut from a metal casing and a plurality of initially-shielded magnets are then used to retrieve the section. The magnets can either be unshielded by a ink mechanism, or pivotally-mounted magnets can be moved toward the cutout section. This type of device, however, requires ongoing user involvement for its operation, and employs a more complicated mechanism than the subject invention.